Current detecting circuit

ABSTRACT

According to one embodiment, a current detecting circuit includes: a normally-ON type first switching element that includes a drain, a source, and a gate; a normally-OFF type second switching element including a drain that is connected to the source of the first switching element, a source that is connected to the gate of the first switching element, and a gate; and a differential amplification circuit that outputs a voltage according to a voltage between the drain and the source of the second switching element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 16/542,697 filed on Aug. 16, 2019 and is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-243301, filed on Dec. 26, 2018; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a current detecting circuit.

BACKGROUND

Conventionally, there has been disclosed a semiconductor device including a normally-ON type switching element and a normally-OFF type switching element that are cascode-connected. For example, the normally-ON type switching element is constituted of a transistor that is made from Gallium Nitride (GaN) and/or Silicon Carbide (SiC). By employing the normally-ON type switching element made from GaN and/or SiC, it is possible to provide a semiconductor device having high withstand voltage and low energy loss. On the other hand, when the normally-ON type switching element is provided, for example, there may present a case where output current of the semiconductor device is not able to be precisely detected in response to leakage current of the normally-ON type switching element. Thus, there has been desired a current detecting circuit having a high reliability, which is able to precisely detect the output current while taking advantage of characteristics of the semiconductor device including a normally-ON type switching element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a current detecting circuit according to a first embodiment;

FIG. 2 is a diagram illustrating characteristics of a normally-ON type switching element;

FIG. 3 is a diagram illustrating a current detecting circuit according to a second embodiment;

FIG. 4 is a diagram illustrating a current detecting circuit according to a third embodiment;

FIG. 5 is a diagram illustrating a configuration example of an adjustment circuit;

FIG. 6 is a diagram illustrating a current detecting circuit according to a fourth embodiment;

FIG. 7 is a diagram illustrating another configuration example of the adjustment circuit;

FIG. 8 is a diagram illustrating another configuration example of the adjustment circuit;

FIG. 9 is a diagram illustrating another configuration example of the adjustment circuit;

FIG. 10 is a diagram illustrating another configuration example of the adjustment circuit;

FIG. 11 is a diagram illustrating a current detecting circuit according to a fifth embodiment;

FIG. 12 is a diagram illustrating a current detecting circuit according to a sixth embodiment;

FIG. 13 is a diagram illustrating a current detecting circuit according to a seventh embodiment;

FIG. 14 is a diagram illustrating a current detecting circuit according to an eighth embodiment; and

FIG. 15 is a diagram illustrating a current detecting circuit according to a ninth embodiment.

DETAILED DESCRIPTION

According to one embodiment, a current detecting circuit includes: a normally-ON type first switching element that includes a drain, a source, and a gate; a normally-OFF type second switching element including a drain that is connected to the source of the first switching element, a source that is connected to the gate of the first switching element, and a gate; and a differential amplification circuit that outputs a voltage according to a voltage between the drain and the source of the second switching element.

Exemplary embodiments of a current detecting circuit will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

First Embodiment

FIG. 1 is a diagram illustrating a current detecting circuit according to a first embodiment. A current detecting circuit according to the present embodiment includes a normally-ON type switching element Q1. The switching element Q1 is constituted of an N-channel MOS transistor made from, for example, GaN. For example, a main current path between a drain and a source of the MOS transistor made from GaN is made of GaN. Hereinafter, the MOS transistor may be referred to as a GaN transistor.

A normally-OFF type switching element Q2 is constituted of an N-channel MOS transistor made from, for example, Si. For example, a main current path between a drain and a source of the MOS transistor made from Si is made of Si. Hereinafter, the MOS transistor may be referred to as an Si transistor. A drain of the switching element Q1 is connected to a terminal 11. The terminal 11 is connected to a power-source line (not illustrated) to which a voltage of 600V is applied, for example. A source of the switching element Q1 is connected to a drain of the switching element Q2.

A source of the switching element Q2 is connected to a terminal 12. To the terminal 12, a ground potential is supplied, for example. A gate of the switching element Q2 is connected to a terminal 10. To the terminal 10, a drive signal V_(G) is applied.

The drain of the switching element Q2 is connected to a non-inversion input end (+) of a differential amplification circuit A1, and the source of the switching element Q2 is connected to an inversion input end (−) of the differential amplification circuit A1. The differential amplification circuit A1 outputs an output voltage V_(C) to a terminal 13.

A current detecting operation according to the present embodiment will be described below. Changing, into High, the drive signal V_(G) that is applied to the gate of the switching element Q2 causes turning ON of the switching element Q2. A drain-source voltage V_(DS) of the switching element Q2 at this time is indicated by a formula (1).

V _(DS) =R _(ON) ×I _(D)  (1)

R_(ON) indicates an ON-resistance of the switching element Q2, and I_(D) indicates an output current I_(D) flowing through the switching element Q1. Current flowing through the switching element Q2 is substantially equal to current flowing through the switching element Q1, and thus the current flowing through the switching element Q2 becomes current that is substantially equal to the output current I_(D). Hereinafter, for convenience of explanation, a drain current I_(D) of the switching element Q1 may be used as the output current I_(D).

The output voltage V_(C) of the differential amplification circuit A1 may be indicated by the following formula (2)

V _(C) =A×R _(ON) ×I _(D)  (2)

where “A” indicates a voltage gain of the differential amplification circuit A1.

For example, the voltage of the drive signal V_(G), which is to be applied to the gate of the switching element Q2, is set such that a relation between a gate-source voltage V_(GS) and a drain-source voltage V_(DS) of the switching element Q2 is “V_(GS)>>V_(DS)”, and the set voltage is supplied to the switching element Q2 so as to turn ON the switching element Q2, so that it is possible to cause the switching element Q2 to operate in a linear region. When the switching element Q2 operates in the linear region, the drain-source voltage V_(DS) of the switching element Q2 is the voltage that is proportional to the output current I_(D), as a result, it is possible to cause the differential amplification circuit A1 to output the output voltage V_(C) that is proportional to the output current I_(D). Thus, it is possible to precisely detect the output current I_(D) when the switching element Q2 is being driven.

According to the present embodiment, the voltage, which is according to the drain-source voltage V_(DS) of the switching element Q2 operating in the linear region, is output by the differential amplification circuit A1, so that it is possible to precisely detect the output current I_(D) flowing into the normally-ON type switching element Q1 when the switching element Q2 is being driven.

FIG. 2 is a diagram illustrating characteristics of the normally-ON type switching element. In other words, characteristics of the above-mentioned switching element Q1 according to the first embodiment are illustrated. In FIG. 2, the gate-source voltage V_(GS) is indicated on the lateral axis, and the drain current I_(D) is indicated on the vertical axis. A characteristic curve 100 indicates that the drain current I_(D) flows even when the gate-source voltage V_(GS) is substantially zero (0) V, and the drain current I_(D) is substantially zero (0) A when the gate-source voltage V_(GS) becomes a minus threshold voltage V_(TH).

Second Embodiment

FIG. 3 is a diagram illustrating a current detecting circuit according to a second embodiment. Note that in the following, a part different from the above-mentioned embodiment will be mainly described, and the common parts are represented with same symbols and the description is omitted appropriately. The same applies hereinafter. Turning ON/OFF of a differential amplification circuit A11 according to the present embodiment is controlled by the drive signal V_(G) that is applied to the gate of the switching element Q2.

Specifically, the differential amplification circuit A11 is in an operation state (ON) when the drive signal V_(G) applied to the gate of the switching element Q2 is High, and is in a non-operation state (OFF) when the drive signal V_(G) is Low. For example, when the drive signal V_(G) is Low, control for turning OFF a bias circuit (not illustrated) of the differential amplification circuit A11 is executed.

The switching element Q1 has a normally-ON characteristic, and thus the drain current I_(D) flows even in a state where a ground potential applied to the terminal 12 is supplied to the gate of the switching element Q1. Thus, when the drive signal V_(G) applied to the gate of the switching element Q2 is Low, in other words, when the switching element Q2 is OFF, a drain voltage V_(X) of the switching element Q2 rises.

According to the present embodiment, when the drive signal V_(G) applied to the gate of the switching element Q2 is Low, the control is executed in which operation of the differential amplification circuit A11 is turned OFF. Therefore, in a case where the drive signal V_(G) is Low, the differential amplification circuit A11 is OFF even when the drain voltage V_(X) of the switching element Q2 has risen, and thus, for example, 0V is output. Thus, it is possible to prevent an erroneous detection of an overcurrent state responding to leakage current of the normally-ON type switching element Q1. Furthermore, control for causing the differential amplification circuit A11 to operate is executed only when the switching element Q2 is in a driving state, so that it is possible to reduce the power consumption.

Third Embodiment

FIG. 4 is a diagram illustrating a current detecting circuit according to a third embodiment. An adjustment circuit 20 according to the present embodiment adjusts a voltage between the drain and the source of the switching element Q2 and supplies the adjusted voltage to the differential amplification circuit A11. The adjustment circuit 20 performs voltage division on, for example, the drain voltage V_(X) of the switching element Q2, and supplies the divided voltage to the differential amplification circuit A11.

The switching element Q1 has a normally-ON characteristic, and thus the drain voltage V_(X) of the switching element Q2 rises when the switching element Q2 is OFF. There presents a case where a high voltage (for example, 600V) is applied to the terminal 11 in some cases while taking advantage of the characteristic of the normally-ON type switching element Q1. Thus, there presents a case where, when the switching element Q2 is OFF, the drain voltage V_(X) of the switching element Q2 becomes a high voltage.

According to the present embodiment, the drain voltage V_(X) of the switching element Q2 is adjusted and the adjusted voltage is supplied to the differential amplification circuit A11. Therefore, applying of overvoltage to the differential amplification circuit A11 is able to be prevented, so that it is possible to prevent a failure in the differential amplification circuit A11 due to the applying of overvoltage and further it is possible to improve the reliability. Moreover, by employing a configuration in which the drain voltage V_(X) of the switching element Q2 is adjusted and the adjusted voltage is supplied to the differential amplification circuit A11, it is possible to configure the differential amplification circuit A11 with a low-withstand voltage, and further possible to simplify the circuit configuration and the manufacturing process of a semiconductor device equipped with the current detecting circuit.

FIG. 5 is a diagram illustrating a configuration example of the adjustment circuit 20. The adjustment circuit 20 according to the present configuration example includes (i) a fixed resistance R1 to one end of which the drain voltage V_(X) of the switching element Q2 is applied and the other end of which is connected to a non-inversion input end (+) of the differential amplification circuit A11; and (ii) a fixed resistance R2 one end of which is connected to the non-inversion input end (+) of the differential amplification circuit A11 and the other end of which is connected to the terminal 12.

The drain voltage V_(X) of the switching element Q2 is adjusted by voltage division, which is performed by the serial circuit of the fixed resistance R1 and the fixed resistance R2, and the adjusted voltage is supplied to the non-inversion input end (+) of the differential amplification circuit A11. Thus, when a voltage-dividing ratio is adjusted by obtaining desired values of resistance values of the fixed resistance R1 and the fixed resistance R2, it is possible to prevent a case where an overvoltage is applied to the non-inversion input end (+) of the differential amplification circuit A11.

Fourth Embodiment

FIG. 6 is a diagram illustrating a current detecting circuit according to a fourth embodiment. An adjustment circuit 30 according to the present embodiment adjusts, in accordance with the drive signal V_(G) applied to the gate of the switching element Q2, the drain voltage V_(X) of the switching element Q2, and supplies the adjusted voltage to the differential amplification circuit A11.

The adjustment circuit 30 executes adjustment in which an attenuation amount of the drain voltage V_(X) to be supplied to the differential amplification circuit A11 is reduced when the drive signal V_(G) applied to the gate of the switching element Q2 is High, in other words, in a driving state where the switching element Q2 is turned ON; and in which the attenuation amount of the drain voltage V_(X) to be supplied to the differential amplification circuit A11 is increased when the drive signal V_(G) applied to the gate of the switching element Q2 is Low, in other words, when the switching element Q2 is OFF.

The output current I_(D) is able to be appropriately detected in a driving state obtained by controlling the adjustment circuit 30 in accordance with the drive signal V_(G) applied to the gate of the switching element Q2, adjusting the drain voltage V_(X) of the switching element Q2, and supplying the adjusted voltage to the differential amplification circuit A11 so as to turning ON the switching element Q2. Hereinafter, a specific configuration example will be explained.

FIG. 7 is a diagram illustrating a configuration example of the adjustment circuit 30. The present configuration example is provided with a variable resistance VR1 whose resistance value is adjusted by the drive signal V_(G), and the fixed resistance R2. A resistance value of the variable resistance VR1 is reduced when the drive signal V_(G) is High, and is increased when the drive signal V_(G) is Low. Hence, when the drive signal V_(G) is High and the switching element Q2 is in a driving state, a control for reducing an attenuation amount of the drain voltage V_(X) is executed, and when the drive signal V_(G) is Low, a resistance value of the variable resistance VR1 is increased to attenuate the drain voltage V_(X) and the attenuated voltage is supplied to the differential amplification circuit A11, so that it is possible to prevent a case where overvoltage is supplied to the differential amplification circuit A11.

When the drive signal V_(G) is High, in other words, in a case of a driving state obtained by turning ON the switching element Q2, a control for reducing an attenuation amount of the drain voltage V_(X) is executed. In other words, when “VR1” is assumed to be a resistance value of the variable resistance VR1 and “R2” is assumed to be a resistance value of the fixed resistance R2, a voltage-dividing ratio [=resistance value of fixed resistance R2/(resistance value of variable resistance VR1+resistance value of fixed resistance R2)] becomes large, and thus an attenuation amount of the drain voltage V_(X) is reduced to be able to supply the attenuated voltage to the differential amplification circuit A11. Therefore, it is possible to output the output voltage V_(C) on which the drain-source voltage V_(DS) of the switching element Q2, generated in proportion to the output current I_(D), is precisely reflected. Thus, it is further possible to precisely detect the output current I_(D) when the switching element Q2 is turned into a driving state.

A resistance value of the variable resistance VR1 is reduced and thus a change in the drain voltage V_(X) is able to be rapidly transmitted to the differential amplification circuit A11, so that it is possible to improve a response speed of the differential amplification circuit A11 to be able to rapidly detect a change in the output current I_(D).

Furthermore, a resistance value of the variable resistance VR1 is increased when the drive signal V_(G) is Low, a voltage-dividing ratio, which is derived from the variable resistance VR1 and the fixed resistance R2, is reduced, and thus the drain voltage V_(X) is attenuated and the attenuated voltage is supplied to the differential amplification circuit A11. Thus, it is possible to prevent a case where an overvoltage is supplied to the differential amplification circuit A11 when the switching element Q2 is OFF.

For example, in a case where a switch (not illustrated) is connected to a resistance in parallel and the switch is turned ON when the drive signal V_(G) is High, a configuration is able to be realized in which a resistance value of the variable resistance VR1 is reduced when the drive signal V_(G) is High.

FIG. 8 is a diagram illustrating another configuration example of the adjustment circuit 30. The present configuration example is provided with the fixed resistance R1, and a variable resistance VR2 whose resistance value is adjusted by the drive signal V_(G). A resistance value of the variable resistance VR2 is increased when the drive signal V_(G) is High, and is reduced when the drive signal V_(G) is Low.

In other words, a resistance value of the variable resistance VR2 is increased when the drive signal V_(G) is High to drive the switching element Q2, a voltage-dividing ratio [=resistance value of variable resistance VR2/(resistance value of fixed resistance R1+resistance value of variable resistance VR2)] of the fixed resistance R1 and the variable resistance VR2 becomes large, so that it is possible to reduce an attenuation amount of the drain voltage V_(X) so as to supply the voltage to the differential amplification circuit A11. Therefore, it is possible to output the output voltage V_(C) on which the drain-source voltage V_(DS) of the switching element Q2, generated in proportion to the output current I_(D), is precisely reflected. Thus, it is further possible to precisely detect the output current I_(D) when the switching element Q2 is turned into a driving state.

Furthermore, a resistance value of the variable resistance VR2 is reduced when the drive signal V_(G) is Low, a voltage-dividing ratio derived from the fixed resistance R1 and the variable resistance VR2 becomes small, and thus the drain voltage V_(X) is largely attenuated to be supplied to the differential amplification circuit A11. Thus, it is possible to prevent a case where an overvoltage is supplied to the differential amplification circuit A11 when the switching element Q2 is OFF.

FIG. 9 is a diagram illustrating another configuration example of the adjustment circuit 30. The present configuration example is provided with the variable resistance VR1 and the variable resistance VR2 whose resistance values are adjusted by the drive signal V_(G). A resistance value of the variable resistance VR1 is reduced when the drive signal V_(G) is High, and is increased when the drive signal V_(G) is Low. A resistance value of the variable resistance VR2 is increased when the drive signal V_(G) is High, and is reduced when the drive signal V_(G) is Low.

By employing such a configuration, when the drive signal V_(G) is changed into High to drive the switching element Q2, a resistance value of the variable resistance VR1 is reduced and a resistance value of the variable resistance VR2 is increased, and a control for increasing a voltage-dividing ratio [=resistance value of variable resistance VR2/(resistance value of variable resistance VR1+resistance value of variable resistance VR2)] derived from the variable resistance VR1 and the variable resistance VR2 so as to reduce an attenuation amount of the drain voltage V_(X) is executed; and when the drive signal V_(G) is Low, a resistance value of the variable resistance VR1 is increased and a resistance value of the variable resistance VR2 is reduced, and a control for reducing the voltage-dividing ratio derived from the variable resistances VR1 and VR2 to increase an attenuation amount of the drain voltage V_(X) and supplying the voltage to the differential amplification circuit A11 is executed.

By employing the adjustment of an attenuation amount of the drain voltage V_(X), which is executed by the adjustment circuit 30 in accordance with the drive signal V_(G), when the switching element Q2 is driven with the drive signal V_(G) being High, it is possible to output the output voltage V_(C) on which the drain-source voltage V_(DS) of the switching element Q2, generated in proportion to the output current I_(D), is precisely reflected. Moreover, when the drive signal V_(G) is Low, the drain voltage V_(X) is largely attenuated to be supplied to the differential amplification circuit A11, so that it is possible to prevent a case where an overvoltage is supplied to the differential amplification circuit A11 when the switching element Q2 is OFF.

For example, the variable resistance VR1 may have a configuration including a parallel circuit constituted of a fixed resistance and a switch (not illustrated), and the variable resistance VR2 may have a configuration including a parallel circuit constituted of a fixed resistance and a switch (not illustrated). A configuration may be employed in which, when the drive signal V_(G) is High, a switch, constituting the variable resistance VR1, is turned ON to reduce a resistance value of the variable resistance VR1, and when the drive signal V_(G) is Low, a switch, constituting the variable resistance VR2, is turned ON to reduce a resistance value of the variable resistance VR2.

FIG. 10 is a diagram illustrating another configuration example of the adjustment circuit 30. The present configuration example is provided with switches S1 and S2 whose turning ON/OFF is controlled by the drive signal V_(G). The switch S1 is turned ON when the drive signal V_(G) is High, and the switch S2 is turned ON when the drive signal V_(G) is Low.

By employing such a configuration, when the drive signal V_(G) is changed into High to drive the switching element Q2, the switch S1 is turned ON and the switch S2 is turned OFF. When the switch S1 is turned ON, a resistance value of the switch S1 is able to be neglected, when the switch S2 is turned OFF, a resistance value of the switch S2 becomes infinite, and thus when the drive signal V_(G) is changed into High to drive the switching element Q2, the drain voltage V_(X) is able to be directly supplied to a non-inversion input terminal (+) of the differential amplification circuit A11 without attenuation.

Thus, when the drive signal V_(G) is changed into High to drive the switching element Q2, the drain voltage V_(X) is directly applied to the non-inversion input terminal (+) of the differential amplification circuit A11, and thus when the drain voltage V_(X) generated in accordance with the output current I_(D) is detected by the differential amplification circuit A11, the output current I_(D), when the switching element Q2 is driven, is able to be precisely detected.

Moreover, a resistance value of the switch S1 when the switch S1 is ON is able to be neglected, and thus a change in the drain voltage V_(X) when the switching element Q2 is in a driving state is able to be rapidly transmitted to the differential amplification circuit A11, so that it is possible to rapidly detect a change in the output current I_(D).

Furthermore, the switch S1 is turned OFF in a state where the drive signal V_(G) is Low, a supply route of the drain voltage V_(X) to the differential amplification circuit A11 is shut. Thus, it is possible to prevent a case where an overvoltage is applied to the differential amplification circuit A11 when the switching element Q2 is OFF.

Each of the switches S1 and S2 may be constituted of a switching element such as a MOS transistor and a bipolar transistor, and turning ON/OFF thereof may be controlled by the drive signal V_(G).

Fifth Embodiment

FIG. 11 is a diagram illustrating a current detecting circuit according to a fifth embodiment. The present embodiment is provided with a comparison circuit C11 that compares the drain voltage V_(X) with a predetermined reference voltage V_(REF). The comparison circuit C11 has a configuration in which a gain of the comparison circuit C11 is higher than a gain of the differential amplification circuit A11, and its response speed is rapid, for example. The comparison circuit C11 may have a configuration without a feedback loop, for example.

The reference voltage V_(REF) is applied to an inversion input end (−) of the comparison circuit C11. The drain voltage V_(X) is supplied, via an adjustment circuit 30-1, to a non-inversion input end (+) of the comparison circuit C11. The adjustment circuit 30-1 has a configuration that is the same as that of the adjustment circuit 30, and may have any one of the above-mentioned configurations illustrated in FIGS. 7 to 10.

When the drive signal V_(G) becomes High and the voltage supplied via the adjustment circuit 30-1 is higher than the reference voltage V_(REF), the comparison circuit C11 supplies, to a terminal 14, an output signal V_(F) that is High. In other words, when a voltage according to the drain voltage V_(X), generated in accordance with the output current I_(D), is higher than the reference voltage V_(REF), the comparison circuit C11 outputs the output signal V_(F) that is High.

The drain voltage V_(X) is proportional to the output current I_(D). Therefore, the comparison circuit C11 compares a voltage, which is according to the drain voltage V_(X), with the reference voltage V_(REF), which is set as a threshold value for detecting overcurrent, to be able to perform the overcurrent detection.

By employing a configuration in which the drain voltage V_(X) is supplied, via the adjustment circuit 30-1, to the comparison circuit C11, it is possible to realize a configuration in which an attenuation amount of the drain voltage V_(X) is reduced and the voltage is supplied to the comparison circuit C11 in a state where the drive signal V_(G) is High and the switching element Q2 is in a driving state, so as to precisely detect the output current I_(D) when the switching element Q2 is in a driving state, and when the drive signal V_(G) is Low and the switching element Q2 is OFF, a state where an overvoltage is applied to the comparison circuit C11 is prevented. Furthermore, it is possible to employ a configuration for executing a control for supplying the output signal V_(F), output from the comparison circuit C11, to a control circuit (not illustrated), and stopping applying of the drive signal V_(G) in response to the output signal V_(F) that indicates an overcurrent state, for example.

Sixth Embodiment

FIG. 12 is a diagram illustrating a current detecting circuit according to a sixth embodiment. The present embodiment is provided with a constant-current source 16 that is connected to a power terminal 15, and a switching element Q3 to which a constant current I_(REF) of the constant-current source 16 is supplied. The constant-current source 16 is constituted of a bandgap circuit (not illustrated), for example. A drain of the switching element Q3 is connected to the inversion input end (−) of the comparison circuit C11, a source of the switching element Q3 is connected to the terminal 12, and the drive signal V_(G) is applied to a gate of the switching element Q3.

The switching element Q3 is formed on a semiconductor substrate (not illustrated) on which the switching element Q2 is formed. The switching elements Q2 and Q3 are formed on the same semiconductor substrate, and thus characteristics of both of the switching elements Q2 and Q3 are able to be matched, so that it is possible to cancel difference between the characteristics, for example.

A size of a gate width of the switching element Q3 is set to 1/N (N: arbitrary positive number) of a size of a gate width of the switching element Q2, for example. In a case of such a setting, when a drain current of the switching element Q2 is “N” times as large as a drain current of the switching element Q3, the drain-source voltages V_(DS) of both of the switching elements Q2 and Q3 become the same. Thus, it is possible to realize a configuration in which a drain voltage of the switching element Q3 is supplied, as the reference voltage V_(REF), to the comparison circuit C11, and thus the comparison circuit C11 outputs the output signal V_(F) indicating High when a drain current flowing into the switching element Q2 exceeds a value that is “N” times as large as a value of the constant current I_(REF).

According to the present embodiment, the output current I_(D), when the drive signal V_(G) is High and the switching element Q2 is in an operation state, is able to be detected by using the output voltage V_(C) of the differential amplification circuit A11, furthermore, a value of the constant current I_(REF) is set as, for example, a threshold current for detecting an overcurrent, so that it is possible to detect, by using the output signal V_(F) of the comparison circuit C11, an overcurrent state in a case where the output current I_(D) exceeds a current value that is “N” times as large as the constant current I_(REF).

Seventh Embodiment

FIG. 13 is a diagram illustrating a current detecting circuit according to a seventh embodiment. The present embodiment is provided with the adjustment circuit 30 that is shared by the differential amplification circuit A1 and a comparison circuit C1. The adjustment circuit 30 may employ any of the above-mentioned configurations illustrated in FIGS. 7 to 10. When the adjustment circuit 30 is shared, it is possible to reduce the number of circuit elements to realize cost reduction.

In the present embodiment, a control for turning ON/OFF the differential amplification circuit A1 and the comparison circuit C1 by using the drive signal V_(G) is not executed. As described above, a configuration is employed in which the drain voltage V_(X) is supplied, via the adjustment circuit 30, to the differential amplification circuit A1 and the comparison circuit C1, and when the drive signal V_(G) is High, attenuation of the drain voltage V_(X) is reduced to supply the voltage to the differential amplification circuit A1 and the comparison circuit C, when the drive signal V_(G) is Low, an attenuation amount of the drain voltage V_(X) is increased or supply routes to the differential amplification circuit A1 and the comparison circuit C1 are shut, so that it is possible to prevent a case where an overvoltage is applied to the differential amplification circuit A1 and the comparison circuit C1.

Eighth Embodiment

FIG. 14 is a diagram illustrating a current detecting circuit according to an eighth embodiment. The present embodiment is provided with the comparison circuit C11 that compares the drain voltage V_(X), which is supplied via the adjustment circuit 30-1, with a reference voltage V_(REF1); and a comparison circuit C12 that compares the drain voltage V_(X), which is supplied via an adjustment circuit 30-2, with a reference voltage V_(REF2).

The drain voltage V_(X) is supplied, via the adjustment circuit 30-2, to an inversion input end (−) of the comparison circuit C12, and the reference voltage V_(REF2) is supplied to a non-inversion input end (+) of the comparison circuit C12. Therefore, when the drain voltage V_(X), which is supplied via the adjustment circuit 30-2, is reduced less than the reference voltage V_(REF2), the comparison circuit C12 supplies, to a terminal 17, a signal V_(F2) indicating High level. Thus, it is possible to detect a state where a reverse-direction drain current flows into the switching element Q2 and the drain voltage V_(X) is reduced to less than the reference voltage V_(REF2) due to a voltage drop generated by the switching element Q2, namely, a reverse-direction overcurrent state. The reference voltage V_(REF2) may be set to an arbitrary negative voltage in accordance with a threshold of a current value that is set as an overcurrent state.

According to the present embodiment, it is possible to precisely detect the output current I_(D) when the drive signal V_(G) is High and the switching element Q2 is in a driving state by using the differential amplification circuit A11, furthermore, to detect a forward-direction overcurrent state and a reverse-direction overcurrent state of the output current I_(D). In the present embodiment, a configuration is employed in which operation states of the differential amplification circuit A11, and the comparison circuits C11 and C12 are controlled by using the drive signal V_(G). Thus, in the configuration, the differential amplification circuit A11, and the comparison circuits C11 and C12 operate only in a state where the drive signal V_(G) is High, so that it is possible to reduce power consumption. Note that, a configuration may be employed in which a control for turning ON/OFF using the drive signal V_(G) is not executed on the differential amplification circuit A11 and the comparison circuits C11 and C12.

Ninth Embodiment

FIG. 15 is a diagram illustrating a current detecting circuit according to a ninth embodiment. The present embodiment is provided with the comparison circuit C11 having a non-inversion input end (+) to which the drain voltage V_(X) is supplied via the adjustment circuit 30-1 and having an inversion input end (−) that is connected to the terminal 12. The adjustment circuit 30-1 may employ any of the above-mentioned configurations illustrated in FIGS. 7 to 10.

The comparison circuit C11 outputs, to the terminal 14, an output signal V_(P) indicating High when a voltage supplied to its non-inversion input end (+) is higher than a voltage supplied to its inversion input end (−), and outputs the output signal V_(P) indicating Low level when a voltage supplied to the non-inversion input end (+) is lower than the voltage supplied to the inversion input end (−). When a direction of drain current of the switching element Q2 is a forward direction, a voltage of the non-inversion input end (+) of the comparison circuit C11 becomes higher than a voltage of the inversion input end (−), and when a direction of the drain current of the switching element Q2 is a reverse direction, a voltage of the non-inversion input end (+) of the comparison circuit C11 becomes lower than a voltage of the inversion input end (−). Therefore, a direction of drain current is able to be determined by a level of the output signal V_(P) of the comparison circuit C11, so that it is possible to use the comparison circuit C11 as a zero-cross detecting circuit.

According to the present embodiment, it is possible to precisely detect, by using the output voltage V_(C) of the differential amplification circuit A11, the output current I_(D) when the drive signal V_(G) is High and the switching element Q2 is in a driving state, furthermore, it is possible to execute a zero-cross detection by using the output signal V_(P) of the comparison circuit C11.

In the above-mentioned embodiment, the configuration is employed in which the differential amplification circuits A1 and A11, each of which outputs a voltage according to the drain-source voltage V_(DS) of the switching element Q2, output the single output voltage V_(C); however, the configuration may output a pair of differential signals of a positive side and a negative side.

The normally-ON type switching element Q1 may be constituted of a Junction Field Effect Transistor (JFET).

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A current detecting circuit comprising: a normally-ON type first switching element that includes a drain, a source, and a gate; a normally-OFF type second switching element including: a drain that is connected to the source of the first switching element; a source that is connected to the gate of the first switching element; and a gate; a differential amplification circuit that outputs a voltage according to a voltage between the drain and the source of the second switching element; and a comparison circuit that compares a voltage according to a drain voltage of the second switching element with a predetermined reference voltage.
 2. The current detecting circuit according to claim 1, wherein a source voltage of the second switching element is supplied to the comparison circuit as the predetermined reference voltage.
 3. The current detecting circuit according to claim 1, further comprising: a third switching element that includes a drain, a source, and a gate, wherein a constant current is supplied to the drain of the third switching element, the source of the third switching element is connected to the source of the second switching element, the gate of the third switching element is connected to the gate of the second switching element, and a voltage of the drain of the third switching element is supplied to the comparison circuit as the predetermined reference voltage.
 4. A current detecting circuit comprising: a normally-ON type first switching element that includes a drain, a source, and a gate; a normally-OFF type second switching element including: a drain that is connected to the source of the first switching element; a source that is connected to the gate of the first switching element; and a gate; a differential amplification circuit that outputs a voltage according to a voltage between the drain and the source of the second switching element; and a first comparison circuit that compares, with a first reference voltage, a voltage according to a drain voltage of the second switching element.
 5. The current detecting circuit according to claim 4, wherein a source voltage of the second switching element is supplied to the first comparison circuit as the first reference voltage.
 6. The current detecting circuit according to claim 4, further comprising: an adjustment circuit that is arranged between the drain of the second switching element and an input terminal of the differential amplification circuit, the adjustment circuit adjusting the drain voltage of the second switching element to supply an adjusted drain voltage to the differential amplification circuit.
 7. The current detecting circuit according to claim 4, further comprising: a second comparison circuit that compares, with a second reference voltage, the voltage according to the drain voltage of the second switching element.
 8. The current detecting circuit according to claim 7, wherein the second reference voltage includes an arbitrary negative voltage.
 9. The current detecting circuit according to claim 4, wherein turning ON/OFF of the differential amplification circuit and the first comparison circuit is controlled by a drive signal that is applied to the gate of the second switching element.
 10. A current detecting circuit comprising: a normally-ON type first switching element that includes a drain, a source, and a gate; a normally-OFF type second switching element including: a drain that is connected to the source of the first switching element; a source that is connected to the gate of the first switching element; and a gate; a third switching element including: a drain; a source that is connected to the source of the second switching element; and a gate that is connected to the gate of the second switching element; a constant-current source that is connected to the drain of the third switching element; a differential amplification circuit that outputs a voltage according to a voltage between the drain and the source of the second switching element; and a comparison circuit that compares, with a drain voltage of the third switching element, a voltage according to a drain voltage of the second switching element.
 11. The current detecting circuit according to claim 10, further comprising: an adjustment circuit that is arranged between the drain of the second switching element and an input terminal of the differential amplification circuit, the adjustment circuit adjusting the drain voltage of the second switching element to supply an adjusted drain voltage to the differential amplification circuit.
 12. The current detecting circuit according to claim 10, wherein turning ON/OFF of the differential amplification circuit and the comparison circuit is controlled by a drive signal that is applied to the gate of the second switching element. 